This invention relates to gas lasers wherein a gas or mixture of gases is excited to form a laser medium which can produce or amplify coherent radiation. More particularly, the invention is directed to gas transport lasers, including pulsed gas transport lasers.
In order to facilitate an understanding of the invention, the invention will be described by way of example in connection with excimer lasers. The invention is directed especially to excimer lasers in which the laser medium is an excimer molecule because the pressures and temperatures, as well as the nature of the gases, encountered in excimer lasers render them difficult to implement structurally and operate for extended periods. The exemplary use of the invention in connection with excimer lasers, however, is by way of illustration only and is not to be interpreted as a limitation of the principles of the invention to excimer lasers. As will become clear, the principles which underlie the invention apply generally to other types of gas transport lasers with less demanding structural requirements and operating characteristics than excimer lasers.
Excimers are molecules formed from two species of atoms or molecules when one (or both) of them is raised to an electronically excited state so that they can combine to form an excited molecule. The reaction is promoted by the presence of a chemically inert substance which removes energy from a colliding pair of atoms or molecules of the different species to produce the excimer molecule. The excimer molecule is an unbound or only very weakly bound molecule in the ground state. This excimer molecule emits radiation at a particular characteristic laser wavelength when it returns to the ground state, the wavelength of the emission from the excimer molecule which is formed being determined from the energetics of the physical system (i.e., the energy difference between the metastable and the ground state of the excimer molecule). Inasmuch as the excimer molecule exists only briefly in the ground state, after formation of the excimer molecule and subsequent emission of radiation, the atoms or molecules return to their original species.
One known type of excimer laser includes a polyatomic excimer laser medium consisting of rare gas atoms and molecules of reactant, which undergo bonding interactions, when the rare gas atoms are excited to metastable states. The molecules of reactant which react with the rare gas atoms, for example, can be monohalide molecules, which react in bimolecular reactions to form the polyatomic excimer. Since the rare gas and monohalide molecules generally return to their original species after each cycle of excitation, emission, and return to ground state, reaction products in the laser chamber are generally eliminated at the conclusion of each cycle.
Detailed descriptions of excimer lasers are found in the literature. See, for example, the article by J. J. Ewing, "Review of UV Excimer Lasers," Ultra High Power Lasers, SPIE, Vol. 76, 1976, pages 132-143, and the bibliographical references cited in the article, hereby incorporated by reference herein.
An important consideration relative to gas lasers is the cleanliness of the laser gases. In order to achieve long-life operation with one laser gas fill, it is necessary to prevent the gas or mixture of gases from becoming contaminated.
However, the existence of highly active reactants is a serious difficulty, especially in rare gas monohalide excimer lasers which use fluorine, chlorine, bromine, or iodine. Not only are the halogens highly reactive and difficult to handle with safety, but residual reaction products caused by contact of halogens with structural materials result in absorbing or scattering species, including particulates, which limit life in rare gas monohalide excimer lasers. It is highly desirable that the gas mixture not come into contact with any material which can form volatile or particulate compounds in the presence of the laser gases. Otherwise, undesired reaction products can be present which deteriorate the gas mixture, reducing its life or requiring circulation or flow of clean gas mixture through the laser chamber. Furthermore, difficulty has been experienced in rendering the laser chamber which contains the gas mixture leak tight.
Contamination of the gas mixture and leakage can reduce the period of operation of known excimer lasers to unacceptably short times. The present invention provides a gas transport laser system in which formation of reaction products which contaminate the gas mixture and leakage of the laser gases are minimized.
Additionally, difficulty has arisen in pulsed gas transport lasers in connection with exciting laser gases to produce lasing action, such as by pulsed electrical discharges. In order to produce a reliable high average power excimer laser, for example, various criteria must be met. One criterion is that in an electrically excited excimer laser, the electrical discharge forming network must be capable of providing high current, high voltage pulses at high pulse rates over a large number of pulses without degradation or failure. In addition, the electrical discharge forming network must be capable of providing a large current rise time (dI/dt). The combined requirements on the electrical discharge forming network components of high average power, for example, high current rise time (.apprxeq.10.sup.11 A/sec), high voltage (.apprxeq.50 KV), high average input power (.apprxeq.5 KW), relatively high pulse rates (.apprxeq.500 Hz), and long life, makes implementation of the electrical discharge forming network difficult.
If the current rise time across the laser electrodes is an appreciable fraction of the electrical discharge pulse duration, however, the electrical discharge forming network can only deliver a small fraction of its stored energy to the gas mixture. The current rise time can be limited by the inductance of the laser electrical discharge current loop which includes the laser electrodes. In known excimer lasers, residual circuit inductance has dominated the current rise time.
Unfortunately, a competing requirement in a high pulse rate excimer laser is to clear the gas mixture from between the electrodes in a time short compared to the interpulse period in order to obtain a uniform electrical discharge. In known fast flow excimer lasers, the gas flow between the electrodes is in a sheet perpendicular to the plane of the electrical discharge. The discharge products and heated gas mixture are carried downstream into the current return conductor. This necessitates repositioning the downstream current return conductor and accompanying insulator so that they are farther from the electrodes, thereby increasing the inductance of the electrical discharge loop, which adversely affects the current rise time.
A low electrical discharge loop inductance is critical in obtaining a uniform, efficient gas discharge. The gas transport laser system in accordance with the present invention minimizes the inductance of the electrical discharge loop.
Furthermore, the gas clearing ratio (i.e., the ratio of the distance the gas mixture travels between electrical discharge pulses to the electrode width in the gas flow direction) must be greater than one. If the clearing ratio, which is typically three to five, in a transverse gas flow geometry is not sufficient, then the electrical discharge tends to follow a path through the hot gas of the previous cycle and not between the electrodes.
In addition, the use of insulation along the current return conductor is limited. The current return conductor and associated insulation must present minimal resistance to the gas flow in order to minimize the power consumed by the blower motor. Also, the upstream conductor must be configured so as to not produce gas density variations (wakes or vortices) in the gas mixture between the electrodes, as this produces electrical discharge non-uniformity.
One embodiment of the gas transport laser system in accordance with the present invention provides a gas flow configuration which can avoid the aforementioned gas flow difficulties. At the same time, the inductance of the electrical discharge loop is minimized.
Another criterion is that in order to obtain efficient, stable operation with minimal electrode wear, a uniform electrical discharge must be produced. Many known excimer lasers incorporate an array of spark sources to produce ultraviolet radiation for preionizing the gas mixture. The spark source has several disadvantages. Firstly, the fact that sparks are used means that some spark electrode erosion occurs. This limits the life of the spark source through physical erosion and the life of the gas mixture through introduction of contaminants. Furthermore, a separate energy source and electrical discharge forming components are required to generate the sparks. Also, a separate circuit is required in order to provide the correct delay between the spark source discharge and the main electrical discharge. Additionally, the ultraviolet radiation produced floods the entire discharge area. The resulting preionization near the electrode edges makes it necessary to contour the edges in order to prevent arc formation there.
The gas transport laser system in accordance with the present invention preferably uses corona preionization for providing electrical discharge uniformity. Consequently, the difficulties involved with spark preionization are avoided.
The gas transport laser system provided by the present invention has the advantage of achieving high average power lasing action. The gas transport laser system is particularly advantageous when configured as a rare gas monohalide excimer laser for producing coherent radiation in the ultraviolet portion of the spectrum suitable for uses such as laser fusion and laser isotope separation and various semiconductor processes.